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Worldwide Travel Alert Issued for Americans

(CNN) The United States issued a worldwide travel alert for Americans amid concerns that terror groups and individuals plan more attacks after the Paris massacres.

The State Department warned that groups like ISIS, al-Qaeda and Boko Haram "continue to plan terrorist attacks in multiple regions."

It also warned of the possibility of individuals carrying out their own attacks.

"These attacks may employ a wide variety of tactics, using conventional and non-conventional weapons and targeting both official and private interests," the department said in issuing a travel alert that expires on February 24, 2016.

The alert does not instruct Americans to avoid travel, but it does urge U.S. citizens to "exercise vigilance when in public places or using transportation."

"Be aware of immediate surroundings and avoid large crowds or crowded places. Exercise particular caution during the holiday season and at holiday festivals or events," the State Department said in the alert.

The department said that terrorist attacks remain likely as ISIS members return from Iraq and Syria.

It warned that "there is a continuing threat from unaffiliated persons planning attacks inspired by major terrorist organizations but conducted on an individual basis," pointing specifically to large sporting events, theaters, open markets and airlines.

In July, the State Department issued a "Worldwide Caution" that also warned ISIS and others continued to plan terrorist attacks against the United States and Western interests in Europe.

An "Alert" is time-limited, often for 90 days, at which it can be reviewed. An alert is not necessarily stronger or weaker than a caution. The most recent worldwide travel alert was issued in December 2014, and prior to that in August 2013, September 2011, and May 2011.